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Aluminum alloy is extremely unstable in the air and easily forms an oxide film that is difficult to detect with the naked eye. Due to different CNC processing methods of aluminum parts, such as casting, forging, extrusion or CNC precision machining, or forming by different processes and then heat treatment or welding, the surface of the workpiece will show different states, with different degrees of dirt or traces. During pretreatment, the method is selected based on the workpiece surface.

1. Issues that need to be paid attention to before anodizing CNC machined parts: Although there is an oxide film on the surface

It doesn't take long and is relatively easy to remove, but it is very greasy, especially in and around the holes (lubrication and additions are required during machine processing). Such workpieces must be cleaned with organic solvents first. If they are washed directly with alkali, not only will they be greasy, but they will be heavier than the removal. Clean, and the finely machined surface cannot withstand long-term alkali corrosion. The result will also affect the roughness of the workpiece surface and the tolerance of the tie, and may eventually become a waste product.

2. Problems that arise during the processing before casting. Not all surfaces of casting molded parts are machined. The unmachined surface leaves a thick oxide layer formed during casting processing. Some clips also have a sand layer. They should be machined first. Either remove the original oxide film of the part by sandblasting, or process it again after alkali cleaning. This can not only remove the original oxide layer of the unprocessed parts, but also avoid changes in the tolerance of the machined parts.

3. Issues that need to be paid attention to during the pretreatment process of the workpiece after heat treatment or welding process: According to process requirements, the workpiece needs to be cleaned with organic solvents before entering the heat treatment or welding process, except for oil stains on the clean surface. However, this is generally not the case at present, so the surface of the workpiece A layer of oil burnt tar is formed. This layer of tar must be removed in an organic solvent. If soaked in alkali solution, it will cause local corrosion, pitting or unevenness, seriously affecting product quality. I use the concentrated nitric acid soaking method for soft coking, and the soft coking material is washed slightly in an alkaline solution.

Some specific methods are as follows.

①Degreasing with organic solvent. If the oil is severe, it can be soaked in solvent in a short period of time; in severe cases, use cotton cloth dipped in solvent to wipe the oil, or scrub with a bristle brush. Operation must be safe and residual solvents must be properly stored after use.

② Let dry. Regardless of organic solvent cleaning, the drying process cannot be omitted, otherwise the meaning of cleaning will be lost.

③Binding. Wire-tying materials are suitable for aluminum wires. Copper wires and galvanized iron wires are prohibited. Return layers can be used. For slightly larger single-sided binding, consider the position of the tie and tie it on the hole closest to the edge of the part as much as possible to reduce the impact on the surface of the workpiece. Different types of workpieces should not be tied to the same rope because different compositions (grades) of aluminum oxide require different processing times. Pay attention to the direction of the shaking of the strip artifact and avoid pitting the concave part downward. Clean the oil stains on the surface of the workpiece.

④Alkali cleaning.

⑤ Rinse with water. It is best to rinse with hot water after washing, which is a good way to clean alkaline substances on the surface of the workpiece. Workpieces with blind holes and groove reinforcements should be flush with each other, and the residual liquid should be poured out, and the nitric acid should be turned over immediately to avoid oxidation.

⑥Nitric acid.

If it is mixed aluminum, cast aluminum or 50mL/L should be added to hydrofluoric acid according to this formula to speed up the removal of alkali attached to the aluminum surface when insoluble matter is removed.

2. Technical requirements for oxide film formation process

(1) Oxidation. Solution composition and working conditions:

After pretreatment, immediately transfer to the oxidation process to prevent the workpiece from remaining in the atmosphere for too long and producing a native oxide layer, which will affect the quality of the oxide layer. Soaking in water is better than exposing it to the atmosphere again, but it should not be soaked for too long. If soaked in 3% dilute nitric acid for L5~30 minutes, oxidation will continue, but if the time is too long, it will affect the film, especially if it contains Old nitric acid with copper and other impurities.

The oxidation temperature of the solution is a key process. If the solution temperature is too high, the film formation speed will be fast, and the oxide film will easily pulverize; if the solution temperature is too low, the coating speed will be slow, resulting in different depths of the coating film and poor adhesion.

To achieve consistent color on the surface of the same aluminum material, it should be processed at the same temperature at the same time.

Within a certain range, temperature is inversely proportional to time, that is, the higher the temperature, the shorter the time required, and vice versa.

High-purity alumina requires longer processing times. The oxidation treatment time is not enough and the oxide film is too pale; the purity and redox of aluminum or the aging of the oxide film may even affect the conductivity of the film.

In order to obtain a uniform paint film color, small pieces of rock oxidized in the solution can be mixed with a large solution, or statically (do not mix the solution, do not shake the workpiece) to prevent the edge of the workpiece from switching with the solution beyond the center of the workpiece, resulting in uneven film color.

(2) Washing circulating water. For blind holes, cut the workpiece, strengthen the flushing of the parts, and dump the residual liquid in the mesh to prevent the oxidation liquid from flowing out and causing oxidation damage.

(3) Self-check. After flushing the circulating water, the workpiece should be inspected for quality. If defects are found in the alkaline solution, they will be returned after slight oxidation. It is more difficult to remove, repair, and dry after stripping and is more likely to damage the substrate.

(4) Dry. Drying is the key to maintaining quality. Shake the free water on the surface of the workpiece before oxidation during the drying process and then expose it to sunlight. It can also be baked at 45~50℃ drying conditions. The temperature should not be too high to avoid burning, aging, cracking and stale appearance.

3. Large area chlorination

(1) Unit. According to the external dimensions of the oxidation (appropriately relax the margin), make a box with wood or bricks, cover the box with a plastic sheet, and form a concave pool. If it is a board, the height is about 100MM. All you need to do is shake this The workpiece on the pool forms an oxide film on its surface.

(2) Parts. The method is implemented in which the parts in the working tank (pool) undergo rapid changes or rotations in different parts, and finally the entire workpiece surface is contacted with the solution multiple times and a thicker oxide film is gradually formed.

The above two methods can be prepared by making a large bath and a large amount of solution, which reduces the waste caused by long-term use and does not occupy the workshop in the production area.

4. Three possible reasons for uneven coloring of the oxide film

(1) The area of the workpiece is too large, the operation swing in the groove is too large, the edge is in contact with the center spot, large updates, and exchange differences lead to inconsistent film colors...

Preventive measures: The oxidized workpiece has a small swing and is fine with static electricity. However, when the temperature is too low, pattern-like spots are prone to appear and appear unnatural.

(2) The aluminum layer in the aluminum processing part is damaged and cut. The outer aluminum packaging is high-quality aluminum, and the inner layer is sealed with a mixture of aluminum. The two kinds of aluminum are different, so "good wind" spots appear after oxidation. Customers often do not understand this phenomenon, and manufacturers should do more explaining to explain the reasons and methods to avoid misunderstandings.

(3) Operational aspects.

① The alkali etching process of parts is incomplete, and the original film and local dirt cannot be removed;

② The alkali corrosion immediately after treatment is not light and the surface is alkaline;

③ The workpiece comes into contact with foreign matter during the transfer process.

These factors are important for manufacturing high-quality anodized aluminum parts. Find the causes of uneven coating colors and take steps to solve them.

5. There are two main reasons why it is difficult to form an oxide film around the hole.

(1) Rinse the workpiece thoroughly after cleaning. When the alkali solution is flushed into the hole, it fails to wash out the alkali solution from the hole and oxidizes it, thereby corroding the perforations around the film.

(2) Apply butter around the workpiece hole. Aluminum self-tapping screw holes are very astringent. Operators often use butter or lye to lubricate them when there is a lack of emulsifier. The butter is difficult to remove.

Solution: (1) Wash again with gasoline before alkali washing, and emulsifier should be added to the lotion;

(3) Alkaline cleaning after workpiece washing.

6. Four points should be paid attention to in the post-processing procedure

(1) Rinse with hot water. Hot water is designed to age the film. However, the temperature and time must be strictly controlled. If the water temperature is too high, the thickness of the paint film will decrease and the color will fade. Similar problems will occur if the processing time is too long. The appropriate temperature and time are:

Temperature 40~50℃ Time 0.5~1MIN.

(2) Dry. It is better to let it dry naturally, wash it with hot water and hang it on the workpiece rack, allowing the free water to flow vertically downward on the work surface. Use a towel to drain the water droplets to the lower corners, and apply this method to dry the color without affecting it, making it appear natural.

(3) Aging process. The aging method can be determined according to the climate conditions. In summer, it can be exposed to the sun. In humid weather or winter, oven baking can be used. The process conditions are:

Temperature 40~50℃ 10~15min.